There are generally known two ways of cutting a steel plate into various shapes. In one method, cutting is started from an end of the steel plate. Another method is “piercing start” in which a through hole is formed at a desired position of the steel plate and cutting is started from the through hole. The latter method, “piercing start” is prevailing in the field of automatic cutting by use of an NC device because a workpiece of a desired shape can be cut out of a steel plate.
In conventional cases where a steel plate is cut with a plasma cutting machine by effecting piercing start, during a piercing operation prior to a cutting operation, in which a through hole is made at a cutting start position by a plasma arc generated from the plasma torch, the metal melted by the plasma arc is blown onto the periphery of the hole so that the molten metal adheres to and deposits on the periphery as dross. The dross may contact the leading end of the plasma torch which has been lowered to start a cutting operation, so that the nozzle is melted and damaged. It also could be a cause of occurrence of a double arc which gives damage to the nozzle so that the cutting quality of the product significantly degrades. Especially where a steel plate having a thickness of more than 6 mm or 9 mm is cut, the adverse effects of the dross formed during the piercing operation are no longer negligible. As a means for protecting the nozzle from the dross produced at the time of piercing, there is known an arrangement in which a shield cap is provided outside the nozzle and an assist gas (secondary gas) is jetted from the space between the nozzle and the shield cap. This means has however proved unsuccessful in avoiding the adverse effects of the dross when cutting a steel plate having a thickness of more than 16 mm.
As an attempt to avoid the adverse effects of the dross, the plasma cutting machine 50 shown in FIG. 4 has been proposed in Japanese Utility Model Publication Kokoku No. 7-26054. The plasma cutting machine 50 has a nozzle 52 disposed beside a plasma torch 51 for jetting a dross adhesion inhibitor for preventing the adhesion of dross. In the plasma cutting machine 50, the following operation is performed: The plasma torch 51 is first moved to a position above the cutting start position of a steel plate W and then, the dross adhesion inhibitor is jetted from the nozzle 52 provided for the plasma torch 51 toward the cutting start position for about 2 seconds. After effecting an operation called “pre-flow” as a preparatory step for plasma arc generation for about 2 seconds, in which plasma gas or the like is jetted from the plasma torch 51 to purge residual gas from the plasma torch 51, a plasma arc is generated from the plasma torch 51 thereby performing a cutting operation started by a piercing start. A film of the dross adhesion inhibitor is thus formed on the cutting start position beforehand so that the adhesion/deposition of the dross generated during piercing can be prevented. In FIG. 4, reference numeral 53 designates a tank for storing the dross adhesion inhibitor. Reference numeral 54 designates a pump for feeding the dross adhesion inhibitor from the tank 53 to the nozzle 52 and reference numeral 55 designates a line for feeding compressed air to the nozzle 52, the compressed air being used for jetting the dross adhesion inhibitor.
Examples of the dross adhesion inhibitor used herein are liquid formulations which are called “laser-non-dross”, formed by dispersing carbon into a solvent and applied to the rear face of an object material for preventing dross adhesion in laser cutting; and solvents which are applied to the front face of a part in the vicinity of a welding area in order to prevent spatter generated in arc welding from adhering to the surroundings. Machine lubricant oils, edible oils and aqueous solutions containing resin may be used as the dross adhesion inhibitor, because solvents containing grease have the effect of preventing dross adhesion.
As techniques associated with the prior art, there have been proposed (i) a plasma cutting machine (see Japanese Patent Publication Kokai No. 6-262367) in which a cooling gas mixing member is disposed outside or inside the plasma torch for generating a gaseous mixture of cooling gas and water and the gaseous mixture of cooling gas and water is jetted to the cut face of the steel plate together with a plasma arc; and (ii) a spatter adhesion preventing method (Japanese Patent Publication Kokai No. 2000-246446) which uses a welding torch designed to selectively supply shield gas and air for use in blowing to a gas passage formed in the welding torch and in which a spatter adhesion inhibitor liquid is applied to the inside of the nozzle of the welding torch and to the contact tube by introducing the spatter adhesion inhibitor liquid into the blowing air in the form of mist.
The plasma cutting machine 50 disclosed in Japanese Utility Model Publication Kokai No. 7-26054 is formed such that the dross adhesion inhibitor is jetted toward the cutting start position from the nozzle 52 disposed beside the plasma torch 51. Even if the dross adhesion inhibitor is jetted during pre-flow, it is interrupted by plasma gas or the like jetted at the time of pre-flow so that the dross adhesion inhibitor cannot be applied to the cutting start position. Therefore, the operation of jetting the dross adhesion inhibitor has to be performed prior to pre-flow as noted earlier. For this reason, a time (about 2 seconds) is required for application of the dross adhesion inhibitor before a start of every cutting operation, which increases the cycle time, resulting in a decline in the productivity. Since the dross adhesion inhibitor jetting position is off to upper right or left of the cutting start position, there is the possibility of missing the target. Additionally, the dross adhesion inhibitor applied to the steel plate W is blown and scattered at the time of pre-flow. In view of these facts, a considerable amount of dross adhesion inhibitor needs to be jetted for reliable formation of a film of the dross adhesion inhibitor at the cutting start position, which involves additional running cost. In addition, since a considerable amount of dross adhesion inhibitor is thus jetted onto the cutting start position, the upper face of the steel plate W in the area around the cutting start position is smudged with excessive dross adhesion inhibitor. Where a dross adhesion inhibitor containing grease is used, it cannot be removed by natural drying and therefore, there arises, in some cases, a need for wiping out the excessive dross adhesion inhibitor after completion of cutting. This requires additional labor. Further, since the nozzle 52 for jetting the dross adhesion inhibitor is off to the upper right or left of the cutting start position, there is a likelihood of occurrence of such troubles that the jet orifice of the nozzle 52 is clogged with molten metal (spatter) scattering at the time of piercing or the leading end of the nozzle 52 is melt down.
In the plasma cutting machine disclosed in Japanese Patent Publication Kokai No. 6-262367, the liquid contained in the gaseous mixture of gas/liquid jetted from the plasma torch is water used for cooling. According to the test made by the inventors, the surface of the steel plate at the cutting start position needs to be covered with a film containing grease or carbon in order to prevent dross adhesion. In view of this, the dross adhesion inhibiting effect cannot be expected from the technique disclosed in Japanese Patent Publication Kokai No. 6-262367 according to which the surface of the steel plate at the cutting start position is covered with water.
According to the spatter adhesion preventing method disclosed in Japanese Patent Publication Kokai No. 2000-246446, since the air for blowing is selected to be fed to the gas passage within the welding torch at the time of initial set-up during off-line and the air for blowing is mixed with the spatter adhesion inhibitor, a reduction in the cycle time cannot be expected. Further, this spatter adhesion preventing method is directed to prevention of adhesion of spatter to the inside of the nozzle of the welding torch and to the contact tube and therefore differs from the present invention in terms of objects.
The invention has been made taking the above problems into consideration and a primary object of the invention is therefore to provide a plasma cutting process and plasma cutting machine capable of reliably applying a necessary amount of dross adhesion inhibitor to a cutting start position of an object material, which leads to prevention of adhesion/deposition of dross, improved productivity due to a reduction in the cycle time, a reduction in the running cost and man hour, and improved reliability.